Antilock control method

ABSTRACT

An antilock control method wherein during the running of an automotive vehicle on a road surface (μ-split surface) having a different coefficient of friction where one of the right and left wheels of the same axle is on a road surface of the higher coefficient of friction and the other one of the wheels is on a road surface of the lower coefficient of friction, when the brake is operated and an antilock control starts, the wheel that runs on the road surface having the lower coefficient of friction excessively slips in a first cycle of the antilock control, and a predetermined condition is satisfied, 1) a hydraulic braking pressure to the wheel that runs on the road surface having the higher coefficient of friction is progressively decreased every period while changing a forcible pressure-decreasing time, and 2) after the pressure decreasing, another predetermined condition is satisfied, a gentle decreasing of the hydraulic braking pressure to the wheel that runs on the road surface having the higher coefficient of friction starts, whereby the road surface having the higher coefficient of friction is efficiently utilized, thereby eliminating the insufficient deceleration of the vehicle body.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to an antilock control method inwhich during the running of an automotive vehicle on a road surface(μ-split surface) having a different coefficient of friction where oneof the right and left wheels of the same axle is on a road surface ofthe higher coefficient of friction and the other one of the wheels is ona road surface of the lower coefficient of friction, when the brake isoperated and an antilock control starts, the braking force differencebetween the right and left front-wheels is reduced, thereby improvingthe vehicle stability and making the steering maneuver by the drivereasy.

[0002] When the running vehicle abruptly brakes, the wheel of thevehicle is locked and skids since there is a limit of frictional forcebetween the wheel and the road surface.

[0003] For the means to prevent the wheel locking phenomenon, there hasbeen known a conventional antilock control system which controls a sliprate of the wheel on the road surface so as to be constant.

[0004] In the vehicle equipped with the antilock control system, whenone of the wheels of the same axle runs on the surface having the lowercoefficient of friction (referred to as a low-μ load surface), e.g., anice road surface, if the wheels are controlled so that the slip rate ofone of those wheels is equal to that of the other one, a braking forceto the wheel on the road surface having the higher coefficient offriction (referred to as a high-μ road surface) becomes larger than thatof the wheel on the low-μ load surface, so that the vehicle possiblyspins. Particularly in those vehicles, e.g., the short-wheel basevehicle, the high gravity-center vehicle, the vehicle having a largescrub radius, the vehicle will more easily spin.

[0005] The solution to this problem is proposed: when the wheel on thelow μ road surface (select low) is locked, the hydraulic brakingpressure to the wheel on the high μ road surface (referred to frequentlyas a “high μ road side wheel”) is forcibly decreased in accordance withthe hydraulic braking pressure to the wheel on the low μ road surface(referred to frequently as a low μ road side wheel) (select low), asdisclosed in Japanese Patent Laid-Open Publication No. Sho. 63-170154.

[0006] The proposed solution in which the brake pressure to the high μroad side wheel is controlled with respect to a state of the low μ roadside wheel (select low), has the following problem, however. When thelow μ road side wheel (select low) slips, the hydraulic braking pressureto the high μ road side wheel is also forcibly reduced in synchronismwith the slippage. Therefore, it is impossible to efficiently utilizethe braking force to the wheel on the high μ road side wheel. In otherwords, an efficiency of the utilization of the road surface on which thehigh μ road side wheel runs is reduced, so that the stopping distance islong.

SUMMARY OF THE INVENTION

[0007] Accordingly, an object of the present invention is to provide anantilock control method which efficiently utilizes the high μ roadsurface, thereby eliminating the insufficient deceleration of thevehicle body.

[0008] To achieve the above object, there is provided an antilockcontrol method wherein during the running of an automotive vehicle on aroad surface (μ-split surface) having a different coefficient offriction where one of the right and left wheels of the same axle is on aroad surface of the higher coefficient of friction and the other one ofthe wheels is on a road surface of the lower coefficient of friction,when the brake is operated and the decreasing of the brake pressure tothe right and left wheels starts, and the wheel that runs on the roadsurface having the lower coefficient of friction excessively slips,

[0009] 1) a forcible pressure-decreasing time of the wheel that runs onthe road surface having the lower coefficient of friction isprogressively changed every period within a first cycle of the antilockcontrol (ranging from a pressure decreasing to the next pressuredecreasing), to thereby reduce an excessive pressure decrease, and

[0010] 2) when a hold mode that follows the end of the forciblepressure-decrease control to the wheel that runs on the road surfacehaving the lower coefficient of friction continues for a time over apredetermined time, a gentle pressure increase control starts (the startof the gentle pressure increase control is quickened).

[0011] The antilock control method of the invention controls thehydraulic braking pressure to the wheel on the high μ road surface whilemonitoring the hydraulic braking pressure to the wheel on the low μ roadsurface. Therefore, the antilock control method can efficiently utilizethe braking force to the wheel on the high μ road surface without anyinfluence on the braking operations of the wheels running on a roadwayhaving a uniform wheel to road condition, and can secure a vehiclestability in the initial braking stage on the μ-split surface.

[0012] According to the present invention, there is provided an antilockcontrol method wherein during the running of an automotive vehicle on aroad surface (μ-split surface) having a different coefficient offriction where one of the right and left wheels of the same axle is on aroad surface of the higher coefficient of friction and the other one ofthe wheels is on a road surface of the lower coefficient of friction,when the brake is operated and an antilock control starts, the wheelthat runs on the road surface having the lower coefficient of frictionexcessively slips in a first cycle of the antilock control, and apredetermined condition is satisfied,

[0013] 1) a hydraulic braking pressure to the wheel that runs on theroad surface having the higher coefficient of friction is progressivelydecreased every period while changing a forcible pressure-decreasingtime, and

[0014] 2) after the pressure decreasing, another predetermined conditionis satisfied, a gentle decreasing of the hydraulic braking pressure tothe wheel that runs on the road surface having the higher coefficient offriction starts, whereby the road surface having the higher coefficientof friction is efficiently utilized, thereby eliminating theinsufficient deceleration of the vehicle body.

SUMMARY OF THE INVENTION

[0015]FIG. 1 is a block diagram showing an antilock control system whichis an embodiment of the present invention;

[0016]FIG. 2 is a graph showing variations of hydraulic brakingpressures to the wheel that runs on the low and high μ road surfaceswhen the antilock control of the invention is performed, with respect towheel speed;

[0017]FIG. 3 is a graph useful in explaining how to decrease thehydraulic braking pressure to the wheel running on the high μ roadsurface;

[0018]FIG. 4 is a graph useful in explaining how to gently increase thehydraulic braking pressure to the wheel running on the high μ roadsurface;

[0019]FIG. 5 is a flow chart showing a forcible pressure-decreasecontrol in the antilock control method which is an embodiment of thepresent invention; and

[0020]FIG. 6 is a flow chart showing a gentle pressure increase controlin the antilock control method which is an embodiment of the presentinvention;

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] The preferred embodiment of the present invention will bedescribed with reference to the accompanying drawings. FIG. 1 is a blockdiagram showing an antilock control system (3-channel brake controlsystem (the front wheels are brake controlled independently and the rearwheels are brake controlled commonly), which is an embodiment of thepresent invention. FIG. 2 is a graph showing variations of hydraulicbraking pressures to the wheel that runs on the low and high μ roadsurfaces when the antilock control of the invention is performed, withrespect to wheel speed. FIG. 3 is a graph useful in explaining how todecrease the hydraulic braking pressure to the wheel running on the highμ road surface. FIG. 4 is a graph useful in explaining how to gentlyincrease the hydraulic braking pressure to the wheel running on the highμ road surface. FIGS. 5 and 6 cooperate to show a flow chart of anantilock control method which is an embodiment of the present invention.

[0022] The antilock control method may be applied to the antilockcontrol systems of type of 4-sensor/4-channel and 4-sensor/3-channel(front dependent control).

[0023] In FIG. 1, reference numeral 1 is a right front-wheel speedsensor; 2 is a left front-wheel speed sensor; 3 is a right rear-wheelspeed sensor; and 4 is a left rear-wheel speed sensor. The signals asfrequency signals output from those wheel speed sensors 1 to 4 arerespectively sent to arithmetic operation circuits 5 to 8 which in turnmathematically process those received signals and produce wheel speedsignals. Of the wheel speed signals output from the arithmetic operationcircuits 5 to 8, a signal indicative of the highest wheel speed isselected by a select high 9. Of the right and left rear-wheel speedsoutput from the arithmetic operation circuits 7 and 8, a signalindicative of the lowest wheel speed is selected by a low select 10. Theoutput signal of the low select 10, and the output signals of thearithmetic operation circuits 5 and 6 are input to control circuits 13,and 11 and 12 byway of signal transmission paths illustrated.

[0024] The control circuits 11 to 13 control, in an antilock controlmode, the on/off operations of hold valves HV and decay valves DV inconventional manners in accordance with the signals received.

[0025] When the automotive vehicle runs on a μ-split surface and itswheel running on the low μ road surface greatly slips in the initialstage of the antilock control, the antilock control method of theinvention varies, every predetermined time period, a forcible pressuredecreasing time of the wheel running on the high μ road surface tothereby suppress an excessive pressure decrease, and quickens a gentlepressure increase of the hydraulic braking pressure to the brake of thewheel on the high μ road surface whereby the high μ road surface isefficiently utilized and hence the insufficient deceleration problem isremoved (this will be described later).

[0026] The antilock control of the invention is substantially the sameas that executed by the conventional antilock control system except thecontents of the arithmetic operations performed by the control circuits11 to 13. Description to follow will be made placing emphasis on thecontrol (referred to as an “antilock control procedure”) executed by thecontrol circuits. Circuits for executing the antilock control proceduremay be provided in those control circuits 11 to 13. In an alternative,those circuits are separately formed in advance and connected to thecontrol circuits conventionally used.

[0027] The antilock control procedure executed by the control circuitswill now be described. During the running of an automotive vehicle on aμ-split surface, the antilock control procedure starts the braking ofthe wheels of the vehicle, and executes special controls (initialpressure-decrease control and a gentle pressure-increase control), whichis different from a normal control. In those controls, one (running onthe high μ road surface) of the right and left front-wheels iscontrolled in accordance with a control state of the other one (runningon the low μ road surface) of the right and left front-wheels.

[0028] Reference is made to FIGS. 2 to 4.

[0029] (1) An initial forcible pressure-decrease control for the wheel(high μ road side wheel) on the high μ road surface starts provided thatthe following conditions a) to d) all hold:

[0030] a) the antilock control is in its first cycle (denoted as X inFIG. 2),

[0031] b) the high μ road side wheel is not in a pressure decreasingmode,

[0032] c) the speed of the high μ road side wheel have never decreasedto below a predetermined threshold value VT2 (having a profileresembling a profile representing a pseudo-vehicle speed Vv) shown inFIG. 2, and

[0033] d) the speed of the low μ road side wheel is below the thresholdvalue VT2 and that wheel is in a pressure decreasing mode.

[0034] (2) The initial forcible pressure-decrease control for the high μroad side wheel ends provided that either of the following conditions e)and f) holds:

[0035] e) the pressure decreasing mode of the low μ road side wheelterminates, and

[0036] f) the high μ road side wheel is placed to the pressuredecreasing mode.

[0037] Where the conditions for 1) above are satisfied, the initialforcible pressure-decrease control for the high μ road side wheel isperformed in the following:

[0038] g) when the speed of the low μ road side wheel is below thethreshold value VT2 (at point A in FIG. 2), the high μ road side wheelis subjected to the initial forcible pressure-decrease,

[0039] h) the initial forcible pressure decreasing rate is set inaccordance with a speed band of the pseudo-vehicle speed Vv,

[0040] i) a set value is subtracted from an initial set value everypressure decreasing cycle, whereby an excessive pressure decrease of thehydraulic braking pressure to the high μ road side wheel is suppressed(the pressure decreasing time is progressively reduced within a fixedtime as shown in FIG. 3),

[0041] j) when the speed of the low μ road side wheel reaches a low peak(point B in FIG. 2), the initial forcible pressure-decrease control forthe high μ road side wheel is terminated, and

[0042] k) if the brake pressure control to the low μ road side wheel isin a hold mode after the speed of the low μ road side wheel reaches thelow peak, the brake pressure control to the high μ road side wheel isalso placed to the hold mode.

[0043] When the high μ road side wheel is under the brake pressurecontrol stated above, the low μ road side wheel slips earlier than thehigh μ road side wheel and is subjected to the brake pressure control inthe brake pressure control for the low μ road side wheel. Therefore, thenormal control is performed.

[0044] A gentle pressure-increase control for the high μ road side wheelin the “antilock control procedure” follows:

[0045] (1) The gentle pressure-increase control for the high μ road sidewheel starts provided that the following conditions 1) and m) hold:

[0046] l) a predetermined time (T in FIG. 2) elapses in a state that thehigh μ road side wheel is in the hold mode following the end of theforcible pressure-decrease control, and

[0047] m) a pressure increasing mode is set up for the low μ road sidewheel within the predetermined time T.

[0048] (2) The gentle pressure-increase control ends provided thateither of the following conditions n) and o) holds:

[0049] n) after the gentle pressure-increase control starts, a gentlepressure-increasing time exceeds a preset value (limiter), viz., thegentle pressure-increasing time in the present gentle pressure-increasecontrol exceeds a limiter, and

[0050] o) a deceleration of the pseudo-vehicle speed Vv exceeds itsdeceleration when the speed Vv exceeds the threshold value VT2 (at pointC in FIG. 2) after the first control cycle for the low μ road side wheelin the pressure decreasing control ends.

[0051] The gentle pressure-increase control for the high μ road sidewheel is as follows:

[0052] p) when the speed of the low μ road side wheel reaches a low peak(point B in FIG. 2), the initial forcible pressure-decrease control isterminated, and the high μ road side wheel is placed to the hold mode inharmony with the control mode of the low μ road side wheel.

[0053] q) this hold mode is executed only for the predetermined time T.

[0054] r) in the gentle pressure-increase control, the pressureincreasing rate is progressively increased by a preset value (as shownin FIG. 4, a pressure increasing time is progressively increased asindicated by t1, t2 and t3, within a fixed time t0).

[0055] s) the gentle pressure-increase control is terminated when thepressure increasing rate that is changed every period or fixed timeexceeds a preset value (limier) or when a deceleration of thepseudo-vehicle speed reaches a preset value,

[0056] t) when the split judgement is continued after the gentlepressure-increase control ends, a conventional yaw moment control isexecuted.

[0057] When the high μ road side wheel is under the above control, thehydraulic braking pressure to the low μ road side wheel is controlled ina normal manner in linking with the slipping of the wheel.

[0058] The “antilock control procedure” thus performed will be describedwith reference with control flows charted in FIGS. 5 and 6. FIG. 5 is aflow chart showing a control flow of the forcible pressure-decreasecontrol for the high μ road side wheel, and FIG. 6 is a flow chartshowing a control flow of the gentle pressure-increase control for thesame. Those controls shown in FIGS. 5 and 6 are described in acontinuous program.

[0059] Reference is made to FIG. 5. Upon start of the control programfor the forcible pressure-decrease control for the high μ road sidewheel (viz., decremental control of the hydraulic braking pressure tothe high μ road side wheel), a step S1 of the control program isexecuted to judge whether or not the antilock control is in the firstcycle (X in FIG. 2).

[0060] If the answer is YES (the antilock control is in the firstcycle), a step S2 is executed to judge whether or not the speed of thehigh μ road side wheel has decreased to below the threshold value VT2.Immediately after the present control is initiated, the speed of thehigh μ road side wheel has never gone to below the threshold value VT2,viz., the answer is NO. Therefore, the next step S3 is executed to judgewhether or not the control of the hydraulic braking pressure to the highμ road side wheel is in a pressure decreasing mode.

[0061] If the control of the hydraulic braking pressure to the high μroad side wheel is not in a pressure decreasing mode, viz., the answeris NO, a forcible pressure-decrease control mode to be described belowis executed.

[0062] A step S5 is executed to judge whether or not the speed of thelow μ road side wheel is below the threshold value VT2 and the pressurecontrol to the same wheel is in the pressure decreasing mode.

[0063] If the pressure control to the high μ road side wheel is in thepressure decreasing mode, viz., the answer is YES, a step S6 is executedto judge if an initial forcible pressure-decrease control flag is OFF.If it is OFF, a step S7 is executed to set the flag ON. And the nextstep S8 is executed to set a pressure decreasing time T1 on the basis ofa speed band of the pseudo-vehicle speed Vv at that time. The pressuredecreasing time T1 is expressed as a function of the pseudo-vehiclespeed Vv as given by the equation (1)

T1=f(Vv)  (1)

[0064] Then, a step S9 is executed to start the initial forciblepressure-decrease control for the high μ road side wheel.

[0065] A step S10 is executed to execute a first pressure decreasing(turn on the decay valve and the hold valve) within the pressuredecreasing time or set time T1.

[0066] When the step S10 ends in its execution and the forciblepressure-decrease control needs to be continued, the “antilock controlprocedure” returns to the start of this program, and the steps S1, S2,S3 and S5 are executed and the step S6 is executed. The answer to thestep S6 is NO (the flag is ON), and then a step S11 is executed to judgeif a pressure-decreasing cycle time T0 (FIG. 3) terminates. Thepressure-decreasing cycle time T0 is set in advance.

[0067] If the pressure-decreasing cycle time T0 terminated, a step S12is executed to calculate the next pressure decreasing time T2 by use ofthe following equation (2).

Next pressure decreasing time T2=present pressure decreasing timeT1−preset value (ΔT)  (2).

[0068] A step S13 is executed to turn on the decay valve and hold valvewithin the preset time and to execute the pressure decreasing.

[0069] This will be further described. As shown in FIG. 3, the ON timeof the decay valve and that of the hold valve are progressively reducedwithin the pressure-decreasing cycle time T0.

[0070] If the answer is YES (the speed of the high μ road side wheel hasdecreased to below the threshold value VT2), there is no need ofapplying the forcible pressure-decrease control to the high μ road sidewheel, and hence the “antilock control procedure” enters the controlphase depicted by the flow chart of FIG. 6. If the control of thehydraulic braking pressure to the high μ road side wheel is in apressure decreasing mode in the step S3, the FIG. 5 control is stoppedand a step S4 is executed to decreases the hydraulic braking pressure tothe high μ road side wheel in a normal manner. If the speed of the low μroad side wheel is below the threshold value VT2 and the pressurecontrol to the same wheel is not in the pressure decreasing mode (stepS5), or if the pressure-decreasing cycle time T0 (FIG. 3) is not yetterminated (step S11), the “antilock control procedure” returns to thestart of the program.

[0071] After the forcible pressure-decrease control shown in FIG. 5ends, the gentle pressure-increase control shown in FIG. 6 is thenexecuted.

[0072] To execute the program of the gentle pressure-increase controlfor the high μ road side wheel shown in FIG. 6, a step S14 is executedto judge whether or not the control for decreasing the hydraulic brakingpressure to the low μ road side wheel ends and the brake pressurecontrol is in the hold mode. If the hold mode is set up, a step S15 isexecuted to turn OFF the decay valve associated with the high μ roadside wheel and turn ON the hold valve associated therewith, and to placethe brake pressure control for the same wheel to the hold mode. A stepS16 then is executed to judge whether or not a state that the brakepressure control to the high μ road side wheel is in the hold modecontinues for a time over the predetermined time T. That is, this stepchecks if the predetermined time T in FIG. 2 elapses.

[0073] If the time T elapses, a step S17 is executed to judge whether ornot the control for the low μ road side wheel terminates. If itterminated, a step S18 is executed to terminate the control for the highμ road side wheel, and the forcible pressure-decrease control for thehigh μ road side wheel ends in its execution.

[0074] If the control for the low μ road side wheel is not yetterminated (step S17), a step S19 is executed to judge whether or not agentle pressure-increase control start flag is in an OFF state. If it isOFF, a step S20 is executed to set that flag ON; a step S21 resets apreset gentle pressure-increase time to an initial value; a step S22starts the gentle pressure-increase control; and a step S23 turns OFFthe decay valve and turns OFF the hold valve within the predeterminedtime.

[0075] If the gentle pressure-increase control needs to be furthercontinued after the end of the step S23, the step S19 is executed again.In this step, if the gentle pressure-increase control start flag is ON,a step S24 is executed to judge whether or not a gentlepressure-increasing cycle time t0 terminates. The gentlepressure-increasing cycle time t0 is set in advance.

[0076] If the time t0 terminated (step S24), a step S25 is executed tojudge the relation given by the following expression, viz., whether ornot the present gentle pressure-increasing cycle time t1 exceeds apreset value t_(lim) (limiter).

Present gentle pressure-increasing cycle time t1≧preset value t_(lim)(limiter)

[0077] The preset value t_(lim) (limiter) is previously set within thegentle pressure-increasing cycle time t0.

[0078] If it does not yet reach the preset value t_(lim), a step S26 isexecuted to calculate the next gentle pressure-increasing cycle time t2by use of the following equation (3). A step S27 is executed to turn OFFthe decay valve and the hold valve within the predetermined time and toexecute the gentle pressure-increase control.

Next gentle pressure-increasing time t2=present gentlepressure-increasing time t1+preset value ΔT  (3)

[0079] This control will be further described. As shown in FIG. 5, theOFF time of the decay valve and that of the hold valve are progressivelyincreased within the gentle pressure-increasing cycle time t0.

[0080] If the gentle pressure-increasing cycle time is reached (thegentle pressure-increasing time t1 reaches the preset value (limiter), astep S28 is executed to judge whether or not a preset decelerationoccurs in the pseudo-vehicle speed Vv. If the answer is YES(deceleration occurs), a step S29 is executed to place the hydraulicbraking pressure control for the high μ road side wheel to the pressuredecreasing mode. If the preset deceleration does not occur in thepseudo-vehicle speed, the “antilock control procedure” returns to thestart in its execution, and executes the control of the FIG. 6 flowchart again.

[0081] If the gentle pressure-increasing cycle time is reached (presentgentle pressure-increasing time t1 reaches the preset value (limiter)(step S25), a step S28 is executed to judge whether or not a presetdeceleration occurs in the pseudo-vehicle speed. If the answer is YES, astep S28 is executed to place the hydraulic braking pressure control forthe high μ road side wheel to the pressure decreasing mode. If theanswer is NO, a step S30 is executed to place the hydraulic brakingpressure control for the high μ road side wheel to the pressureincreasing mode (yaw moment control), and here the gentle pressureincrease control is completed.

[0082] If the pressure decreasing control for the low μ road side wheelterminates and is not in the hold mode (step S14), and if the hold modeof the high μ road side wheel continues for a time shorter than thepredetermined time T (step S16), the “antilock control procedure”returns to the start of the control program, and the gentle pressureincrease control is executed again.

[0083] If the preset deceleration occurs in the pseudo-vehicle speed(step S28), the pressure control for the high μ road side wheel isplaced to the pressure decreasing mode (yaw moment control), and thecurrent gentle pressure increase control ends.

[0084] In the antilock control method, wherein during the running of anautomotive vehicle on a road surface (μ-split surface) having adifferent coefficient of friction where one of the right and left wheelsof the same axle is on a road surface of the higher coefficient offriction and the other one of the wheels is on a road surface of thelower coefficient of friction, when the brake is operated and thedecreasing of the brake pressure to the right and left wheels starts,and the wheel that runs on the road surface having the lower coefficientof friction excessively slips,

[0085] 1) a forcible pressure-decreasing time of the wheel that runs onthe road surface having the lower coefficient of friction isprogressively changed every period, to thereby reduce an excessivepressure decrease, and

[0086] 2) the start of the gentle pressure increase control for thewheel that runs on the road surface having the lower coefficient offriction is quickened, whereby the antilock control method canefficiently utilize the braking force to the wheel on the high μ roadsurface, to thereby eliminating the insufficient deceleration problem.

[0087] As seen from the foregoing description, the invention reduces anexcessive pressure decreasing by changing the forcible pressure-decreasevalue for the high μ road side wheel every period. Further, theefficiency of utilization of the high μ road surface is improved byquickening the starting time of the gentle pressure increase control. Alimiter is used for the pressure increasing rate, to thereby prevent anexcessive increase of the hydraulic braking pressure to the high μ roadside wheel which occurs when the gentle pressure increase control isperformed and hence to supress an excessive difference of braking forcebetween the right and left wheels. Further, in the forciblepressure-decrease control for the high μ road side wheel which isperformed in linking with the low μ road side wheel, the invention caneliminate the insufficient deceleration, which is caused by the forciblepressure decreasing for the high μ road side wheel, while being freefrom a slip time of the low μ road side wheel. Therefore, the brakingforce to the wheels can effectively used under every road conditions,and a stable vehicle running is secured.

What is claimed is:
 1. An antilock control method wherein during therunning of an automotive vehicle on a road surface (μ-split surface)having a different coefficient of friction where one of the right andleft wheels of the same axle is on a road surface of the highercoefficient of friction and the other one of said wheels is on a roadsurface of the lower coefficient of friction, when the brake is operatedand an antilock control starts, the wheel that runs on the road surfacehaving the lower coefficient of friction excessively slips in a firstcycle of the antilock control, and a predetermined condition issatisfied, 1) a hydraulic braking pressure to the wheel that runs on theroad surface having the higher coefficient of friction is progressivelydecreased every period while changing a forcible pressure-decreasingtime, and 2) after the pressure decreasing, another predeterminedcondition is satisfied, a gentle decreasing of the hydraulic brakingpressure to the wheel that runs on the road surface having the highercoefficient of friction starts, whereby the road surface having thehigher coefficient of friction is efficiently utilized, therebyeliminating the insufficient deceleration of the vehicle body.
 2. Theantilock control method according to claim 1 , wherein said forciblepressure decreasing condition is satisfied provided that the followingconditions are all satisfied: a) the antilock control is in its firstcycle, b) the high μ road side wheel is not in a pressure decreasingmode, c) the speed of the high μ road side wheel have never decreased tobelow a predetermined threshold value VT2, and d) the speed of the low μroad side wheel is below the threshold value VT2 and that wheel is in apressure decreasing mode.
 3. The antilock control method according toclaim 1 or 2 , wherein said gentle pressure-increasing start conditionis satisfied if either of the following conditions is satisfied: a) apredetermined time elapses in a state that the high μ road side wheel isin the hold mode following the end of the forcible pressure-decreasecontrol, and a) a pressure increasing mode is set up for the low μ roadside wheel within the predetermined time (T).